text:
What are these lines on the fuselage of a 747?
Have a look at this picture:
It's from Airliners.net (though I've modified it a lot) and shows some lines on the fuselage. What are they? Are they Lightning Diverter Strips like in this question ?

aren't they just edges?

Duplicate? Why are these rivets on the 777 so visible?

No, they are just the boundaries between skin panels. See below for a picture of the 747-8 fuselage during assembly.
[
Boeing 747-8 front from side (picture source)
And this is how the panels for the cylindrical section look like before they are joined:

Boeing 747-8 fuselage panels. The original picture (source) is named "wing panel skins", but these are really fuselage parts.

These lines are skin laps. The vertical ones do not overlap, the horizontal laps overlap by three rows of fasteners.

The downvote wasn't mine but it would probably help if you explained what a "skin lap" is. The asker surely wanted to understand what they are, not just what their name is.


text:
Are the implications of weight and balance for rotary wing aircraft similar to fixed wing aircraft?
Original discussion here.
How does weight and balance for rotary wing aircraft work?  Is lateral CG more or less of an issue, or is it comparable to fore/aft CG since the lifting surface is rotating around the aircraft?
What are the major differences compared to fixed wing, if any?

I don't understand your statement since the lifting surface is rotating around the aircraft - not sure where you are going with that.
In general, the principles are the same as for fixed wing.  If the CofG limits are exceeded, it is possible to run out of control authority to the extent that the aircraft becomes uncontrollable.
Many helicopters, I believe any with a tail rotor, have different left/right CofG limits.  The tail rotor opposes the rotation of the fuselage due to torque from the rotor.  Since it does this by producing thrust to the side, there is a "translating tendency" which would cause the helicopter to drift in the direction in which the TR is generating lift.
To counter this, the cyclic neutral position is rigged to the left (CCW rotor) or right (CW rotor) which results in the skid on that side hanging low.  Clearly, there is now less possible movement in the cyclic to that side and therefore, a narrower lateral CofG.
For example, in a helicopter with a counter-clockwise rotating rotor, the fuselage wants to rotate clockwise.  The tail rotor will produce a thrust to the left to counter this and a long lever couple is formed with the main rotor which, if not corrected, would cause the helicopter to drift to the right.
With a perfectly balanced helicopter, in a no wind steady hover, you want the cyclic to be in the centre with no force required from the pilot.  To stop the drift to the right, the cyclic is moved to the left.  To prevent the need for the pilot to do this, the cyclic is rigged so that its neutral position is to the left.  The main rotor thrust vector is now titled to the left which introduces a horizontal component of thrust acting to the left.  This also causes the left skid to hang low.  This is why in most CCW rotor helicopters, the captain sits on the right and in some of these helicopters, solo flight from the left is prohibited.
Since the cyclic is already left of centre, there is less cyclic authority to the left than to the right.  Therefore, the right limit of lateral CofG is less than the left limit.
This could not be achieved by moving the CofG since this would level the fuselage but would not oppose the thrust.
I will never forget the sensation of my first solo lift into the hover after the talking ballast (my instructor) got out and left me to it.  I lifted with the control positions where I was used to them.  Because there was 170lbs less weight in the front left corner, this meant that I started to pitch up and move back.  I was surprised by how much forward and left I had to move the cyclic to counter it.  It felt very odd for a few minutes.

i would think that because a helicopter can hover, therefore lateral cg plays a much bigger role than longitudinal compared to fixed wing, all flight vectors being equal.  in general during forward flight, i expect the effects of w&b on rotary wing to approximate those of fixed wing.

to counteract "translating tendency", instead of trimming the cyclic left or right, would moving the lateral cg left or right also do the trick?

I edited my answer.

@Federico edit his post if it's that important to you.

for the record, @Simon, i don't think I have ever used full cyclic deflection in the R-22, and I don't think I would ever want to.

@rbp Too right. I shudder to think about the attitude you would be in before this, and the mess afterwards :)  You raise a good point though that the CofG limits don't allow you to get anywhere near the cyclic authority limits.

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sort: votes


text:
Where to download NAT Track Message (original and official source)?
I am studying about North Atlantic tracks.
I know the message is usually distributed to interested parties via AFTN and is published by Shanwick or Gander. I would also like to get such information from the same source.
Where can I download NAT Track Messages in their original format (I guess it is the AFTN Message Format)?
Note: I want the official and original message possibly. I don't want other websites that show me NOTAMs or other formats that are not official. I am interested in the original message.

This is a duplicate of Where to download NATs (North Atlantic Tracks) information? You mustn't add a new post when you actually need to edit your initial question. Please use the edited on ... link at the bottom of your initial post.

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text:
What is the difference between published and unpublished waypoints?
While reading some articles about free route airspaces I found reference to two types of waypoints:

Free route airspace (FRA) is a specified airspace within which users
  can freely plan a route between a defined entry point and a defined
  exit point, with the possibility of routeing via intermediate
  (published or unpublished) waypoints ...

My question is the following: What is the difference between published and unpublished waypoints? How are they created? When are they used?
Could you please provide me documentation/citations as well in order to have a complete overview of the topic?

When doing long sensor survey runs, in places like DC and NYC, I use unpublished waypoints to facilitate ATC coordination, and provide a memo in advance to ATC, usually via FAX.  That way they know where I need to fly, and can handle the sequencing to get me there.  Helps with airspace waivers as well, sent to the FSDO in advance.

Published waypoints are waypoints that appear on the charts and in the published navigation databases used by GPS navigators or FMSes.  They are referenced using the published identifier for the waypoint. Examples: TPA, ITAWT.
Unpublished waypoints are waypoints created by the pilot.  FMS allow for the creation/entry of a waypoint by the pilot.  The waypoint is defined by one of the following:

Latitude/Longitude
Place/Bearing/Distance
Place-Bearing/Place-Bearing

The pilot-defined waypoint can then be added to the flight plan.
Here's a snip of how a waypoint is defined in an A320:

As to when you would use it, the short answer is when you need to or want to.  A common example would be to deviate a short distance off an airway (maybe 5 or 10 nm) to avoid a temporary flight restriction.  You would just want to avoid the restricted airspace and using a published waypoint or waypoints would take you farther off your route than necessary.

An example of when today would be http://tfr.faa.gov/save_pages/detail_4_3634.html You may plot a "way point" to go around that little bubble. Instead of going to V152 then to V537 if you traveling on V533. It's a bad example, but an easy to see one.

You didn't mention if you're asking about a specific country or jurisdiction (although your question seems to be based on European information), but here's some information about the US situation.
A published waypoint is simply what it sounds like: it's a waypoint that's published on a chart, approach procedure etc. and has a name. This question has more information.
An unpublished waypoint is one that doesn't exist on any public chart. They're used by ATC or pilots. For the ATC side you can read more in the ATC Orders 4-1-5, but essentially an unpublished fix is simply any uncharted point that ATC uses to direct aircraft, such as an ad hoc holding point, a reporting point or something else:

An unpublished fix is defined as one approved and planned for
  publication which is not yet depicted on the charts or one which is
  used in accord with the following: [...]

It then goes on to essentially say that ATC can define their own fixes as needed for operational reasons. This (fictional) instruction is using an unpublished fix:

N12345, hold east of the Hinch VOR on radial 080 at 10 DME

Note that the orders imply a preference for published fixes:

Unpublished fixes should not negate the normal use of published
  intersections. Frequent routine use of an unpublished fix would
  justify establishing a fix.

From the pilot's side, a pilot can define his own waypoints for a single flight or as a 'permanent' waypoint in the GPS or FMS. Gerry's answer has a nice example from the A320 FMS but for private pilots, tools like ForeFlight or Garmin Pilot allow you to create and save your own waypoints. That can be useful for a single flight if you want to route around weather, a TFR or whatever. Or if you frequently fly the same route, or use the same private (uncharted) airstrips then creating your own personal waypoints can be very convenient.


text:
How much data is stored in the flight data recorder per hour in a typical flight of an A380?
One drawback or impediment to the storage of flight recorder data on the cloud is the amount of data that'd be transmitted and at cost.
How much data in GB or TB is stored in the flight data recorder per hour in a typical flight of an A380?

I don't have the numbers for the A380 but have some for the Dreamliner. On a typical flight a Dreamliner will record about a half a terabyte of data:

This level of operational insight will involve generating large amounts of data from each 787 aircraft, he explained. “We can get upwards of half a terabyte of data from a single flight from all of the different devices which are internet connected," Bulman said.

source
Depending on the airline this data is stored/processed/compressed/uploaded to HQ.
The flight data recorder or "Black Box" stores far less parameters as only 88 are required by the FAA:

(1) Time;
  (2) Pressure altitude;
  (3) Indicated airspeed;
  (4) Heading--primary flight crew reference (if selectable, record discrete, true or magnetic);
  (5) Normal acceleration (Vertical);
  (6) Pitch attitude;
  (7) Roll attitude;
  (8) Manual radio transmitter keying, or CVR/DFDR synchronization reference;
  (9) Thrust/power of each engine--primary flight crew reference;
  (10) Autopilot engagement status;
  (11) Longitudinal acceleration;
  (12) Pitch control input;
  (13) Lateral control input;
  (14) Rudder pedal input;
  (15) Primary pitch control surface position;
  (16) Primary lateral control surface position;
  (17) Primary yaw control surface position;
  (18) Lateral acceleration;
  (19) Pitch trim surface position
  (20) Trailing edge flap or cockpit flap control selection
  (21) Leading edge flap or cockpit flap control selection
  (22) Each Thrust reverser position (or equivalent for propeller airplane);
  (23) Ground spoiler position or speed brake selection
  (24) Outside or total air temperature;
  (25) Automatic Flight Control System (AFCS) modes and engagement status, including
  autothrottle;
  (26) Radio altitude;
  (27) Localizer deviation, MLS Azimuth;
  (28) Glideslope deviation, MLS Elevation;
  (29) Marker beacon passage;
  (30) Master warning;
  (31) Air/ground sensor (primary airplane system reference nose or main gear);
  (32) Angle of attack ;
  (33) Hydraulic pressure low (each system);
  (34) Ground speed ;
  (35) Ground proximity warning system;
  (36) Landing gear position or landing gear cockpit control selection;
  (37) Drift angle
  (38) Wind speed and direction ;
  (39) Latitude and longitude
  (40) Stick shaker/pusher
  (41) Windshear
  (42) Throttle/power lever position;
  (43) Additional engine parameters
  (44) Traffic alert and collision avoidance system;
  (45) DME 1 and 2 distances;
  (46) Nav 1 and 2 selected frequency;
  (47) Selected barometric setting
  (48) Selected altitude ;
  (49) Selected speed
  (50) Selected mach
  (51) Selected vertical speed
  (52) Selected heading
  (53) Selected flight path
  (54) Selected decision height
  (55) EFIS display format;
  (56) Multi-function/engine/alerts display format;
  (57) Thrust command
  (58) Thrust target
  (59) Fuel quantity in CG trim tank
  (60) Primary Navigation System Reference;
  (61) Icing
  (62) Engine warning each engine vibration
  (63) Engine warning each engine over temp.
  (64) Engine warning each engine oil pressure low
  (65) Engine warning each engine over speed
  (66) Yaw trim surface position;
  (67) Roll trim surface position;
  (68) Brake pressure (selected system);
  (69) Brake pedal application (left and right);
  (70) Yaw of sideslip angle;
  (71) Engine bleed valve position
  (72) De-icing or anti-icing system selection
  (73) Computed center of gravity
  (74) AC electrical bus status;
  (75) DC electrical bus status;
  (76) APU bleed valve position
  (77) Hydraulic pressure (each system);
  (78) Loss of cabin pressure;
  (79) Computer failure;
  (80) Heads-up display
  (81) Para-visual display
  (82) Cockpit trim control input position-pitch;
  (83) Cockpit trim control input position--roll;
  (84) Cockpit trim control input position--yaw;
  (85) Trailing edge flap and cockpit flap control position;
  (86) Leading edge flap and cockpit flap control position;
  (87) Ground spoiler position and speed brake selection; and
  (88) All cockpit flight control input forces (control wheel, control column, rudder pedal)

Source
Some modern airliners such as the A330neo store up to 1200 parameters, making it easier to traceback what caused a crash.
Exact numbers are difficult to provide as the data from a black box is considered to be highly confidential.

Note the data size is not only a function of how many parameters but also how frequent they are recorded. Example: Pinpointing airspeed each second requires much more memory than recording it each ten seconds.

I strongly doubt this data (at least in its entirety) is uploaded anywhere. That would be a huge burden on satellite internet suppliers.

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text:
What is the Specific Fuel Consumption of CFM56-7B24/26/27 engines?
I think the title completely says what I need. I use FSX (Flight Simulator X) to simulate flights and while flying the B738 I saw that it consumes much more fuel than the estimated amount, which is calculated by the software. By doing some research in the airplane folder I figured out that the problem is because the wrong Specific Fuel Consumption (SFC) is  entered there. And by some research on the net I also found the type of engine that B738 uses. So, right now, I need to know the SFC of the CFM56-7B24/26/27 engine so that I can enter the correct value in FSX.

Does it vary by engine type installed? Rolls Royce, GE, Pratt & Whitney, etc? Or by the power setting used? Or the altitude that is used for level flight? I know the power setting and altitude sure affect my fuel burn in my little, by comparison, Lycoming O-360.

Related thread elsewhere: http://www.airliners.net/forum/viewtopic.php?t=750059

SFC for CFM56-7B is 0.5lb/lbf/hr. As an aside, the LeapX is 16% better at 0.42lb/lbf/hr.

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text:
How does washout affect the wing incidence required?
In theory: if I wanted to build an model aircraft with (zero zero) wing/tail incidence, how would washout affect the root incidence required?
E.g. if a wing has 2 degrees washout at the tip, would the root have 1 degree + incidence so that the 'average' angle of the wing is zero ?

That depends on the taper ratio.
In the end, the wing with washout should produce the same lift as the one without. For a rectangular wing and 2° washout, your reasoning is right and the root should have 1° incidence so the whole wing produces the same lift as the one without washout and 0° at the root. If the wing has a triangular planform, however, the tip contribution to overall lift is much lower and the root incidence should only be 0.667°.
For a general rule, pick the position along the span at which half of the wing area is inboard of this position and pick the root incidence such that the local incidence at this position is 0°.

Excellent - many thanks for clear explanation.


text:
What is the relation between cargo smoke and EDTO?
I have heard a lot of people discussing about cargo smoke and Extended Diversion Time Operations (EDTO) together. Just want to know how these two affect each other.

Extended Diversion Time Operations https://www.icao.int/SAM/Documents/2014-EDTO/Draft%20EDTO%20Handbook%20edited%20only%20for%20training%20(2).pdf


text:
Why nitrogen generation system is only present in centre tank only?
I was going through FCOM of A320 and it mentioned that nitrogen generation system is only there for centre tank and not for wing tanks what is the reason for the same?

The inerting system was added to the A320 as a result of a new FAA regulation in 2008:

On 21 July 2008, FAA required
operators and manufacturers
to incorporate a Flammability
Reduction Means (FRM) or Ignition Mitigation Means (IMM)
on fuel tanks having a flammability exposure exceeding certain
thresholds [...]

(Airbus FAST Magazine 44)
Airbus found that only the centre tank was affected by these thresholds required by the FAA:

AIRBUS COMPLIANCE TO THE RULES
Airbus demonstrated that ‘only
the centre tank’ of some of its
existing aircraft has fleet average
flammability exposure exceeding
7% and is affected by the
requirements of the FAR Part 26.
There is no necessity to do any
modification on other tanks.
It concerns the following aircraft:

A320 Family,
A330-200, A340-200,
A340-300, A340-500, A340-600,
A300-600.

(Airbus FAST Magazine 44)
What exactly does fleet average flammability exposure mean? The FAA defines it as follows:

the percentage of the flammability exposure evaluation time
(FEET) each fuel tank ullage is flammable for a fleet of an airplane type operating over
the range of flight lengths in a world-wide range of environmental conditions and fuel
properties as defined in this appendix[, which] means the percent of time each fuel tank ullage is flammable for a
fleet of an airplane type operating over the range of flight lengths.

(FAA Advisory Circular 25.981-2A)
Since the centre tank is usually used first, it will spend more time empty (or almost empty). Fuel vapour is highly combustible (much more than liquid fuel), so the risk is significantly higher for the centre tank.

By the way, the system is not really a nitrogen generation system. It only enriches the nitrogen content of some HEPA filtered bleed air to reduce the oxygen content:

Air Separation Module (ASM) description
It is the core of the Inert Gas Generation
System. The objective is to reduce
the centre wing tank ullage O2
concentration to below 12%
during most conditions. Each ASM
is a semi-permeable hollow fibre
membrane bundle contained in a pressure
containment canister. This canister
is a cylinder with three ports. There is
only one ASM on the A320 Family aircraft,
two on the A330-200 and A340-200/300,
and three on the A340-500/600.

(Airbus FAST Magazine 44)

Is that canister something expendable that gets replaced frequently?

@davidbak According to Airbus it needs to be replaced after 27,000 hours, so not really that frequently ;)

Given the image and the replacement interval it's almost certainly basically the same thing you see in an "oxygen concentrator" which you commonly see used by those with breathing difficulties.  I use the quotes because they really are nitrogen concentrators, the nitrogen is discarded and the patient gets the nitrogen-depleted waste air.

Reading the Wikipedia article on oxygen concentrators, it looks as though the Airbus Air Separation Module uses the membrane separation method.  The home oxygen concentrators use the pressure swing adsorption method, which is different.  (I'd guess it's cheaper but less efficient than the membrane method.)

Worthy of note is the proximity of the centre fuel tank to the air conditioning packs.  On some aircraft this can cause additional heating of the centre tank in high temperatures with packs running to cool the cabin for long periods of time.


text:
Why is maximum zero fuel mass used in take off mass calculation?
In  calculation we consider maximum takeoff weight , maximum landing weight and maximum zero fuel weight
we selected the lowest of below mentioned

Maximum Zero fuel weight + Take off fuel
Regulated take off mass
Regulated Landing mass + Trip fuel

"2" and "3" are related to structural limitations that if we cross we might damage the aircraft or runway while doing landing or takeoff . My understanding is since both Regulated take off weight and Maximum Zero fuel weight is related to structural limitation during take off so Maximum Zero fuel weight would be factor while calculating the Regulated take off weight itself. Then why does we consider Maximum Zero fuel weight separately while calculating take off mass?

The calculation that needs to be made is: when trip fuel is subtracted from the MTOW, is the Max Zero Fuel Weight exceeded? MZFW is a structural design limit as well. The weight difference between the MTOW/MLW and the MZFW may only be utilised for addition of fuel.
From Torenbeek, Synthesis of Subsonic Airplane Design:

The empty tank case can be a critical loading case in certain critical areas of the structure at positive load factors, as there is no relieving load due to the fuel mass. For some aircraft there may be a limit to the weight for initiating fuel transfer between tanks (Maximum Design Fuel Transfer Weight. MFTW) .

Picture above is also from Torenbeek page 269, and lists the components of the various weight groups. For instance Operational Empty Weight + Payload = Zero Fuel Weight, Zero Fuel Weight + Fuel At Takeoff = Takeoff Weight etc. All operational actual weights must be below the maximum values imposed by the aircraft manufacturer.

Doesn't maximum take off weight include maximum payload and Max fuel on board ?

Not necessarily...

Then can you explain more about Maximum take-off weight

MTOW is a performance limitation, the total mass the engines have to push, and the wings have to lift.  MZFW is strictly a structural limitation that limits the bending applied to the spars by the weight of the payload in the fuselage that has to be born by the wing spar roots.  Load above MZFW can only be fuel in the wings that is not adding to that spar root bending load.


text:
How to become a helicopter pilot?
I once had a dream of becoming a helicopter pilot. Then life happened: married young, kids, school, career, etc. I'm working in a good career while finishing a Ph.D. in engineering. But still, I find myself wondering "what if?"
My dad was in a similar situation when he was my age (33) when he started flying (and selling to pay for the habit of flying) powered parachutes. I soloed for the first time in a PPC at 10-years-old and have hundreds of unlogged flight hours. Not a helicopter, but still, some flying experience and lots of good memories.
So my question that isn't career advice: what are the paths for getting into a helicopter flight career? And, are there any routes for people like me?

Military? Probably too late for me. I've heard that there are age restrictions but I'm not certain regarding the specifics. Others may be interested, though.
Local flight school? I'm hesitant. I know people who have gotten burned by the flight schools at local airports when the school closed and the owners disappeared with their money. These seem sort of like a Ponzi scheme anyway, IMO, since once you become an instructor, you train others, who are also becoming instructors, who are also... turtles all the way down...
Commercial Flight School?

Are there any grey-beards out there who can offer some perspective on the three options? Or offer other alternatives?

If you don't pay-in-advance, I'm not sure how a flight school can take your money. Many flight schools get payments at the end of a lesson.Also being an instructor doesn't mean they don't have additional training, getting your license and becoming a CFI are two completely different things.

Welcome to aviation.SE! This is a good question but it's also difficult for us to answer because there are so many possible opinions and experiences. This site is based around Q&A rather than discussion, and you might get a better response in an actual discussion forum where you can get multiple opinions, ask follow-up questions etc. It would also be helpful to know which country you're in. But I think you may be overly sceptical about flight schools: they're how most pilots learn to fly, after all (at least in the US), and even commercial schools use their own students as instructors.

@Pondlife I am in the US and am happy to be proven wrong about flight schools; I was just relating my tangential experience.

You are not too old to join the Army as a Warrant Officer, and your type of expertise and life experience is viewed as a virtue. Plus, right now you are more likely to get picked up than at any time I've seen before. There are two factors which are in your favor significantly. First is the fact that every single one of our military service branches are experiencing a severe shortage of pilots due to the airline pilot labor market conditions. Everyone wants to get out and live that dream. Second, did you see the DoD budget passed last week? Every service is plussing up, to include pilots!

If you really want to become a helo pilot, just begin a search for a flight school which does helicopter training.  I do recommend Precision Helicopters out in McMinville, OR, but there are plenty of good schools, all with fine instructors, out there.
As with any kind of flight training, it requires three things:
Commitment:  Once you start your training, be prepared to commit to finishing it.  Flying will teach you a lot about your capabilities and limitations as a human being.  Some areas are going to come easily to you, others you are going to struggle with.  But you can persevere if you work hard at it.  It is imperative for success here that once you start, you do not terminate your training or have extended delays or absences from your flying.  Flying aircraft is a perishable skill and lessons learned can erode away if not regularly practiced.
Time:  be prepared to budget at least three months, maybe more depending on how easily you progress, to flight training.  This will encompass 2-3 flights a week with classroom prep on the side.  This is an ideal amount of time as it keeps your flying regular to maintain proficiency in skills learned while allowing you to fully digest and ruminate on previous lessons.  It also gives extra time should you encounter learning plateaus or other difficulties in your training.
Money:  Unfortunately, no bucks, no Buck Rogers.  Flying is expensive and that goes doubly so for helicopters.  For a private pilot certificate with a helicopter class, I would anticipate spending an estimated \$20,000-\$25,000 on your training.  That will encompass all of your costs, including dual flight time, supplies, ground school, simulator work, FAA written exams and the practical exam (checkride) in the helicopter.
The military is another good option for flight training.  However most service branches require that you having been commissioned as an officer before your 29th birthday in order to be eligible for their flight training programs.  You will also need good eyesight and be in excellent physical condition.

Hey Carlo, the US Army is up to 33 years old with waivers for older. Those waivers are heavily influenced by the education and experience of a candidate. Also, eyesight can be up to 20/50 correctable to 20/20.    http://www.usarec.army.mil/hq/warrant/prerequ/woft.shtml

Yeah there are waivers here and in certain cases more accessible than others.  The military is desperate for pilots right now, as are the airlines, corporate and other flying businesses.

I started at a local flight school when I was 33, my wife 18 months or so later.
Find a school, pay as you go if you like, that's what I did, about $600/monthm trying to get in 2 lessons a week. Took us both 10 months or so with weather delays, work delays, buying a 2 seater with a couple of coworkers, having a kid.  Did a few trips in the 2 seater after my son was born a couple months before I got my license, then bought a 4 seater a little over a year later, and got an instrument license after that so we could reliably get past the clouds that occur over the low mountains west of us.  Go for it - find a flight school that is close to work, or home, and start doing it.


text:
Why are aircraft turbine blades made of nickel alloys?
Why are aircraft turbine blades made of nickel alloys? What is the advantage of using them? Why did they replace steel alloys for turbine blades?
How to overcome failure cases of turbine blades?

Welcome to aviation.SE! I've edited your question a little to make it clearer, but I don't know what you mean by "overcoming failure cases"? Can you explain that part? It might be better as a separate question, depending on what you're asking. You might also want to check out some existing questions that cover this topic.

The answers here do a good job of addressing the aerospace requirements. What makes nickel especially useful would be an interesting question for the Physics or Chemistry sites.

There is a very nice video by Rolls Royce about their engines here on youtube: https://www.youtube.com/watch?v=VfomloUg2Gw

Not all turbine blades are made of nickel alloy, some blades are made of such family of alloys but clearly not all. Some blades are actually made of ceramic materials.
The turbine is always located after combustion chamber and the temperature which the first stage of turbine blades is able to resist is a proof of high technology engine. Those blades are exposed over to 2000ºC; the melting point of steel is around 1100ºC, and nickel can go much further up to 1700ºC. Ceramic can go even further. Note that there is also a significant difference between external flow temperature and surface temperature.
Using a high melting point material is one way to solve the problem, there are also ways to reduce the effective temperature over the surface of the metal like using film cooling.

In the figure I introduced above you can see holes in the blade, where "cooling air" (maybe at 400ºC) flows out of the blade creating effectively a film that goes around the blade protecting (cooling) it from external temperature.
However a metal exposed at a high temperature (below the melting point) is not behaving in the same way as at normal temperature. There are some phenomena affecting the materials like creep. When a material is exposed during long term to high temperatures close to the melting point and high stress at the same time (this is important) it starts suffering deformations that lead to a failure of the metal.
So nickel alloys, apart for having a higher melting point are also having one of the best behaviours in creep environments.
The last stages of the turbine rotors, and more easily stators (less demanded in stress) are less exposed to creep conditions and are not needed to be manufactured in a nickel base alloy.
Just to add as well, in order to reduce creep another approach followed (combined with previous) is to solidify the metal ensuring having a single crystal metal (instead of being a polycrystalline metal). This is utmost art of material engineering (from my point of view).

Ah yes, those cool, refreshing 400°C breezes.

Their main advantage is higher strength at elevated temperatures. With elevated I mean temperatures up to 1200°C, which is what you will find in modern, film-cooled high-performance turbines where the gas temperature is even a few hundred centigrades higher than that. Steel would melt and other materials like titanium would quickly oxidize, and only nickel alloys or ceramics are up to the job. Being still too brittle for everyday use, ceramics have not managed to escape the laboratory, so nickel alloys is all we have for now.
Failures are overcome by shutting the engine down and scheduling it for repair. When a turbine blade fails, the engine will develop an imbalance that might cause secondary damage due to vibrations. A single blade loss can be tolerated for a while, especially when power setting is reduced, but should be repaired as soon as possible.


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What are the most common reasons an aircraft to depressurize while flying?
What are the most common reasons that cause an aircraft to depressurize while flying? For example, a broken window, a failure on the Outflow Valve, etc.

Without listing the probably hundreds of reasons, it boils down to the failure of a component that seals the inside pressure of the aircraft with the outside pressure. The failure can be caused by many factors from fatigue through sabotage.

@RonBeyer or failure of the pressurization pump though that is rarer (I haven't heard of any other than helios 522 and that was because of a miss-set switch).

@ratchetfreak Even if the pump fails, the pressure has to leak back out somewhere, even through the pump. The pump itself acts as a "seal" in that it only allows a one-way movement of pressure, when it fails and the pressure backflows, its a seal failure in a sense :)

@RonBeyer the air would still go out the outflow valves.

Unless they were closed, @ratchetfreak

Speaking of closed... I tried to edit this to make it less broad. Pressure issues do happen and the typical causes should be answerable.

The DC-10 catastrophic decompression accidents were due to improperly designed doors which did not close properly.

You may narrow your question to few types of aircrafts, possibly only one type. Components from one type to another are differents and may not present the same failing rates.

Some of possible causes: cracked windshield, door or window seal problems, rupture on the fuselage, outflow valve malfunction.

The most common failure leading to depressurization I've personally seen is a pack/air cycle machine failure. The equipment that takes pressurized bleed air from the engine, cools and expands it. I've seen it happen two ways. 1) The aircraft is dispatched with one pack on MEL and the second pack fails in flight or 2) The aircraft is flying at high altitude, 1 pack fails, the other pack switches to high flow mode and fails soon after.
Pack failure is by far the most common I've seen. But have also seen a couple failed fuselage skins and a few burst PAX windows. Typically door leaks are enough to be noisy as heck, but not depressurize the aircraft.


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Why are aircraft external lights round, not tear-drop shaped?
I saw this image of a Cessna 172's tail posted on another question and it made me wonder why the beacon light is basically a cylinder, and not a more aerodynamic, teardrop shape?

Image source

Biggest reason would be light spread.

I thought about that, @ratchetfreak, but I would think that a clear housing would help, or, with the current lighting technology, the lens could be shaped on the outside with the appropriate bits on the inside to ensure proper light distribution.

light distribution through a lens is much more complicated than that

Cessna 172's non retractable landing gear probably would have added much more drag than the lights.

Note that the two small antennas you see in the photo, would probably be of similar aerodynamic problem value, than, the shaping of the beacon.

@LieRyan: So? The entire fuselage adds more drag than the lights. Doesn't make it pointless to discuss the lights.

That beacon lens (and most of the strobe lenses shaped like it) are actually Fresnel lenses.
They are optically designed to produce a specific plane & intensity of coverage, and tinted for a specific color. (The color, intensity, and plane are specified in the FARs & applicable Technical Standard Orders if you want to go hunt down the requirements.)
The lens optics are easier to see on the split (Red/White) lenses:

A Fresnel lens is not the only way to meet these requirements, some anti-collision lights do it by sheer intensity (while retaining the "jar" shape, mainly for installation compatibility), and as GdD pointed out there are more aerodynamic options available. The Fresnel lens method is usually adequate for smaller/slower aircraft however, and the drag penalty at relatively low speeds isn't significant enough for designers to worry about.

Your assumption that all are round is faulty, plenty are teardrop shaped.
You'll see round ones on many slower aircraft where streamlining isn't as big an issue, however on faster aircraft they are almost always aerodynamic.

It's interesting that some lights on small aircraft are streamlined (wingtip lights come immediately to mind), but others are disgustingly draggy (like the beacon in the question).

@voretaq7 The wing is critical to lift so makes sense that you'd think hard while putting anything there that'd disturb the flow.

As this just popped up in my feed again, I was rereading it and your understanding of my assumption was faulty. My OP was "Cessna 172... the beacon light" there was no blanket assumption that all GA aircraft lights are round and not aerodynamic. The question was about this particular light on that particular model. Anyway, back to letting sleeping dogs lie...

Taking a look to the photograph you are showing not only the lights are not very aerodynamic, you can see as well that the fairing on the tail could have a softer transition, the rivets used are not the ones used in commercial het airplanes and transition between plates is also not smooth. You can see that the aerodynamics is open to improvements.
The answer is overall airplane optimization.
Cessna 172 is an small airplane of 3 passenguers and one pilot. Customers of this airplane will be mainly of 2 kinds:

Small airlines making short flights between 2 places very close.
Private use.

Notice that although the airplane has an autonomy of 1200km, usually is not able to compete in routes of that distance as airplanes like A320 will be able to offer a cheaper ticket. Is a typical airplane for travels between islands with low traffic.
In that context...

For the airline, as there will no be a big traffic, the main cost will be amortization of the airplane, also trips between 2 points will be usually very short, so the most part of the use of the airplane will be in ground taxing the airport.
For the private use, it will be an airplane not used regularly and the flying time, in the life of the airplane, will be really limited.

In the overall context of the lifetime of the airplane the fuel consumption (driven by the aerodynamic drag) will be smaller than the effect of the amortization of the airplane (capital costs) and maintenance.
In order ot reduce aerodynamic drag and fuel comsuption in general (weight reduction), that will imply extra costs on each manufactured airplane:

Add-hoc design of components not using standards.
Higher quality standards and control.
Higher costs of development (NRC).
High costs materials.
More costly manufacturing shapes.

Basically the manufacturer (Cessna) needs to make a decision between:

Making a relatively cheaper airplane with higher fuel consumption.
Making a costly airplane efficient in fuel consumption

In that trade-off, for this specific model and my understanding of the customer objective, is more attractive to create a cheaper airplane.


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Have the results of the USAF's $AVE program research been implemented?
According to this Aviation Week article from October 2012, the US Air Force Research Laboratory ran a program called Surfing Aircraft Vortices for Energy ($AVE) to test/prove that autopilot settings could be adjusted to allow C-17s to fly in formation using the wingtip vortices of the lead plane to reduce fuel consumption of the following planes. They seem to have found upwards of a 10% reduction in fuel for the following planes.
The article indicated that previous experiments involved hand-flying of the aircraft in formation which required the equivalent concentration, focus and exhaustion levels of aerial refueling and was unfeasible for long duration flights, therefore the tests proposed to incorporate the formation flying into the autopilot
Does anyone know if these auto-pilot configurations have been put into use in line C-17s or other USAF transport aircraft?

So far I found no evidence that line C-17 receives this modification to the autopilot system. However the test aircraft had this modification and it proved satisfactory.

The C-17 has a special autopilot system including a component called the MILCAS-FR formation flying system by Honeywell and here is a link a sales brochure.
My military flight time was not in a C-17 but in my opinion once the precise formation locations were determined that achieve the fuel savings in the article it would be a simple decision to use them on any given formation flight based on the tactical situation.  The requirements of each mission would drive that decision.

So it sounds like this close "vortex surfing" routine has been built into the autopilot software, and its use is determined on a mission-by-mission basis. Excellent, welcome to Aviation.SE, and thanks for serving! Your screen name indicates you may have flown something a bit faster and more nimble than a transport?


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Is a smartphone required to control a DJI Phantom 2 Vision+?
Not sure if this is on-topic here, as the official guidance seems a bit vague.
Anyways, I've got a relatively simple question.  I'm considering purchasing a Phantom 2 Vision+, but am a little unclear about what options there are in terms of controlling it.  Their website makes a big deal about being able to set waypoints (using an iPad/mobile app) and have the quadcopter operate as an autonomous drone.  And that's useful for certain things, but it seems like it would be more fun to control it directly by remote.
Judging from the video here, it does appear that the quadcopter ships with a remote.  But it also looks like you're expected to stick an iPhone/smartphone onto the remote?  Is that a requirement or just an optional extra?
Is it possible to fly the quadcopter without using a smartphone/mobile app (using just what's included in the box)?

Although the guidance is vague, I think this edges towards off topic: your question is about UAVs and hardware, not directly aviation itself. That said, I'm voting to leave open (or rather, not voting to close) on the basis that there are plenty of questions about equipment on full size aircraft which relate to maintenance, rather than actually flying, and we're not so swamped with questions that we can't separate our Cessna's from our UAVs. This is very much on the borderline, though, and it may be worth starting a discussion post on there?

I'm pretty sure it works without the smart phone. If I understand correctly, the smart phone app simply relays the imagery from the camera, which can be useful for navigating especially if you do not have a clear line of sight.
Ars Technica ran a piece on this very model, which might interest you.

Here are your answers:
You can standard control for the P2V+ is via the physical remote it comes with, you can also set waypoints via the app (You may take over control at any time)
To see what you are filming and to start and stop recording among many other features, you will want to have the app on a smartphone connected. Everything you need to do this is included when you buy. You don't have to use the app, but I dont see why not.
Please check out my video for a full review of its features.

The phone/ipad is used to relay video only and other wise control the camera. You also get some flight information such as distance, elevation, speed relayed to your display. It is not a requirement to use it but if you are never going to use it, why not get a straight Phantom 2, which doesn't have a camera on it. It's much cheaper. The camera system on the Phantom 2 vision+ is very impressive though!


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Would it be more effective to increase the length or chord of a rotor blade for better performance?
Would it be more effective to increase the length or chord of a helicopter rotor blade for better performance? (Say, 10%?)

What kind of performance are you looking for? More lift, higher speed, less fuel consumption?

What would best way to make a helicopter quiet? Does the blade length and width affect this?

@PMAN Suggest you ask that question on its own.

It's not so easy. The blade length, chord and engine power are connected in complex ways, and for each desired performance there is an optimal combination of those parameters.
Rotors (and wings) generate lift by accelerating air downwards. The lift is proportional to the rate of change of inertia of the air. This, however, also changes kinetic energy of the air and therefore requires power. This is called induced drag.
Since inertia is proportional to velocity, but kinetic energy is proportional to the square of velocity, it is more efficient to accelerate a larger amount of air to a lower speed.
The amount of air affected is proportional to the area of the rotor disk (the whole area in which the blades move) and that is simply ($\pi\times$) square of blade length. So in a sense the answer is easy: only blade length has effect, blade chord does not.
However, increasing the blade length also increases the form and skin drag of the blades, so beyond some point the form drag will exceed the induced drag and any additional increase in blade length won't improve efficiency further.
Even more importantly, a longer blade will be heavier and therefore will have to be stronger to withstand the centrifugal force when turning, and stiffer so it does not bend too much when not. But that will make it even heavier and you'll reach a point where you simply exceed the capabilities of the material. And of course higher weight also means more lift is required. These effects will further reduce the optimal length of the blades.
The blade chord is then a secondary parameter governed by two concerns: the chord has to be long enough so the blades can generate the necessary lift without stalling and they have to be strong enough, strength being proportional to chord times thickness.
So if you want to improve load, you can try to improve efficiency, which you could do by making the blades longer, but you need to find a material with higher specific strength, otherwise the increased weight will outweigh the improved aerodynamic efficiency.
If you don't have that, you will primarily have to increase power of the engines. The optimal blade length may then be longer too, but you will be hitting the limits of the material quickly; that's why there are multi-rotor helicopters.
If you instead meant speed, that's a completely different issue and neither blade length nor chord play a significant role there. In forward motion, the speed of a helicopter is limited by two constraints: sufficient portion of the retreating blade has to move aft relative to the airflow and the tip of advancing blade must not exceed speed of sound (actually, critical Mach number, which is somewhat lower). The first requirement means the tip speed must be around three times higher than the maximum forward speed, otherwise the retreating side of the rotor won't be able to generate enough lift and the aircraft will roll to that side (actually, aft due to gyroscopic effect). Together with the second parameter, it limits the speed to about a quarter of the critical Mach. There are some experimental twin rotor (intermeshing, side-by-side) helicopters that don't mind the lift asymmetry and can fly somewhat faster. But it has nothing to do with blade length; blades of larger rotor spin at lower rpm to maintain similar tip speed.

Helicopter performance, as in power required for:

Maximum altitude
Hover ceiling
Climb speed
Maximum speed

Points 1. 2. and 3. boil down to the question: how can we maximise thrust for a given installed engine power? This is done by increasing blade span and decreasing rotor solidity (blade chord and number of blades), analogous to having a higher aspect ratio for a given wing area in a fixed wing. There is a limit for this: in the hover, we would not want to exceed critical Mach number since we'll then be senselessly using engine power for overcoming compressibility drag. The answer for points 1. 2. and 3.: blade length.
Item 4. is the odd one out, now we want to decrease blade span and increase blade solidity in order to retain lift. Again the critical Mach number is the limiting factor, now because the tip speed is added to the forward velocity of the helicopter. A single rotor disk is tilted forward to produce thrust, and must also still produce all the required lift. Relief of rotor disk loading enables higher speed, by either:

A smaller rotor disk tilted forward more, in combination with a fixed wing, makes the rotor function more like a propeller and allow for a greater forward speed.
Using counter-rotating dual rotors allows for shorter blades per rotor.
Using separate propellers for thrust, like an autogyro.
Tilting the rotors, so at high speed a fixed wing provides the lift and the outsize propellers provide the thrust.

The answer for point 4.: blade chord, but preferably more blades, or another rotor, or an additional fixed wing.
Image source
As of last year, the speed record for a pure single rotor helicopter was held by the Westland Lynx, using special high-speed wing tip technology to postpone the critical Mach number.
Exceeding 200 knots is a rare experience for a pure helicopter driver. Tilt rotors can do this routinely. This compound helicopter is built for speed as well.
Image source

I assume you are specifically asking about increase in lift.  There are a lot of other constraints and problems that come into play when designing an airfoil, but here is the short answer.
Keep in mind the main limiting factor is going to be the drag on the airfoil.  There are two primary kinds of drag: form drag and surface drag.  Form drag is the drag caused by the area and shape of an object moving through the air - think of this as holding your hand out the window of your car with the palm facing forward.  The other kind of drag is surface drag, this is the drag caused by the surface area parallel to the airflow - think of this as holding your hand out the window with your thumb forward.
Lift is proportional to surface area of the airfoil.  Increasing the size of the airfoil is going to increase the surface drag regardless of weather you increase the length or width.  However, increasing the length of an airfoil is going to introduce more form drag, while increasing the chord is "only" going to increase the surface drag.
So the short answer to your question is: increasing the chord is more efficient than increasing the length.  However, there is obviously a lot more at play, or else helo rotors would be short and stubby!

You forgot the most important form of drag, the induced drag. Which happens to completely turn the equation around, because it is inversely proportional to span.

Besides the blades can't be analysed in isolation and for the whole system the important parameter is the rotor disk area and blade chord does not affect that.


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Which map projection is typically used on ATC radar screens?
Displaying a (roughly) spherical surface on a flat map turns out to be pretty difficult. There are many different map projections, each with their advantages and disadvantages. Some of the most common ones are cylindrical (Miller), conical (Lambert) and azimuthal.

On aviation maps and charts, I often see the Lambert conical projection used. This made me wonder: on an ATC radar screen, which projection is typically used? It is important that the controller can easily estimate both headings and distance by looking at the screen. For large sectors especially, an inaccurate projection might make this difficult.
Clarification: The discussion of whether the choice of projection will have an operational impact at all is an interesting one. My question, however, is not about how important the choice of projection is, but rather what projections are commonly used. I have a professional interest in this, even if the choice of projections is not operationally important.

Maybe different standards are used in different companies or different regions of the world. If that is the case, a brief summary of the most commonly used standards would be appreciated.

How large would a sector have to be before the inaccuracies become operationally significant?

Depends on which projection is used ... hence my question :)

Stereographic, according to this document. More on stereographic. At least the projection must preserve angles (must be conformal) because different radars will see the target and the different displays (or portions) must be consistent.

Looks suspiciously like Eurocat...

I don't think there is a need for any projection, because radar output is not spherical.

Normally stereographic projection is used for ATC displays, especially if a multisensor tracker fuses data from multiple sources. The European surveillance data processing system ARTAS uses it internally and for output (unprojected WGS84 is also supported for displays doing their own projections & controller support tools). The Thales Eurocat/TopSky system also uses stereograhpic projection.
In older single radar systems, sometimes a simple local projection is used.
$X=\rho \sin(\theta)\\
Y=\rho \cos(\theta) $
With:

$\rho$: measured range
$\theta$: measured azimuth

Due to distortion caused by projection from 3D to 2D this only works for a single radar.

Single-sensor SR radars are interesting because 2D radar plots do not contain altitude information, and plots are sometimes projected without height information.
3D SR radar plots can be corrected by taking height information (found by use of eg stacked beams, or even correlation with SSR Mode C data) into account.
In a modern, multi-sensor "data fusing" environment, one can expect some kind of coordinate transformation into more "normal" maps such as Mercator etc, so that controllers can (also) perform accurate range and azimuth measurements.
(I'm a former fighter controller.)

I can only speak by experience, and this is relevant to PALLAS installed in Greek ACC and LGAV approach. What I concluded while discussing with ATCOs is that the projection used is gnomonic. Unfortunately I haven't read any document to prove it (this is pure observation) and even if I had I couldn't cite it (for obvious confidentiality-related reasons).
Anyway, gnomonic is the projection I've used while developing the PALLAS simulator for HCAA and the results were identical. So what characteristics has the gnomonic projection:

Every straight line you draw on it, is part of great circle. This is of great importance since great circle arc between 2 points is the smallest distance between those 2 points. And when flying you want to avoid extra miles
It heavily distorts areas of the map away of the projection center, but gives a very good approximation close to the center. That makes it suitable for relatively small geographical ares.
You can never see a whole hemisphere. If you have a look at the article in Wikipedia, you will see why.

You might want to take a look at this picture which is not from the real system but the simulation. Unfortunately I don't have photo of the system that shows the concept, but I hope this does. Note the 19th and the 30th meridians that are straight lines (every meridian is a great circle arc) and how they converge to the north. Also note the blue-ish line measuring a random distance. This is guaranteed to be a great circle distance. So every straight line on the map, either from a tool like the one pictured or an airway, is a potential trajectory for an aircraft to follow. That way you see the "truth" each time you see a straight line.

Copyright: own work, tool DARSSY
PS: Now regarding your actual question of what projections are commonly used: others have mentioned stereographic while I mention gnomonic (and I'm pretty sure it is this one) for PALLAS. So there is no such thing as commonly. Projection is a tool. And as such you have to pick the right one for each situation. Now I don't know what your professional interest is, either a pilot, ATCO or a fellow developer wanting to write his/her own simulator (or even CWP???) in any case I would suggest you contact someone that knows the internals of the specific system you are interested in.
Regarding accuracy, you first need to address radar accuracy which is affected by coverage, track distance from the radar, atmospheric conditions and other factors that fall beyond the scope of the answer. Then you can wonder if it's worth the pain to analyze if the projection might trick the controller into errors.

It might be useful to note that the map in your picture appears to be centered on the Aegean Sea. Took me a while to recognize the coastlines. (And I'm still not sure what the other less twisty white lines are; Greek airspace boundaries?)

Downvoter cares to explain?

Looks like @mins is correct. According to Stereographic Projection of Radar Data in a Netted Radar System by J.J. Burke,

In netted air defense and air traffic control systems, data from the long range radars are routed to a Sector Operations Center and stereographically projected onto a common coordinate plane for presentation to system operators on the display consoles.

Also, from On the Application of Stereographic Projection to the Representation of Moving Targets in Air Traffic Control Systems by Robert G. Mulholland:

an ARTCC is serviced by a multitude of radars, and control of aircraft in the horizontal sense is effected through stereographic representations of target locations in a single plane.

And,

Horizontal separation of aircraft under the control of a single Air Route Traffic Control Center in the National Airspace System (NAS) is accomplished by controlling the relative separation of points in a plane that represent actual aircraft locations. Such a representation is supposed to be the image of the orthogonal projection of an aircraft onto the mean sea level surface of the earth under a stereographic mapping.

Not sure if this is what he is after, this is about the projection of aircraft onto the map not the physical world onto the screen. The "common coordinate plane" seems to be what he is looking for.

Map projections are notoriously bad at the edges of the area they represent.
Hence, when looking at a wall map of the globe, the area near the middle is fairly accurate, and near the top and bottom it gets screwy.
ATC Sector maps are always shown centered on their space.  So the middle of their map is accurate, and, at worst, the extreme edges are a little distorted.
The biggest Centers I can find are about 1,000 miles from the center to an edge. (The Sectors will obviously be significantly smaller).
So while I cannot tell you the exact amount of distortion, I think the distortion over an estimated 500 miles on a scope would be pretty small.

Thanks, you make an interesting point. However, my question is not about how significant any distortion is, but about which projection is actually used

@J.Hougaard Does it matter though? For the short distances displayed on radar screens, unless you are near the "edges" for that particular projection, they should all be fairly similar. My guess is that because they need to be accurate with the paper maps they use the same Lambert Conformal Conic Projection system used by paper maps.

@RonBeyer It matters to me, which is why I asked the question :) If you are not interested, feel free to ignore it

@J.Hougaard Its an interesting question and it may benefit from the why, the only reason I can think of is to know the distortion over one side of the display to the other, which is why I said its probably insignificant. If you are trying to stitch together all the radar screens over the US, it would give more context. It may be the only way to know is to ask the manufacturer, I'm guessing even ATC operators don't know the answer for sure, I'm just guessing...

As for why, the question came up the other day in class when we were discussing map projections. None of our ATC instructors were able to answer, so I thought I would ask here. Nothing more to it :)

The projection used is simply not relevant.  Horizontal separation of two dots on the screen is easily done, no matter the projection. Any errors in the radar system will be equal for both dots.  As far as exact position over a point on the ground, test flights and/or operational experience will position them close enough.

I have some experience here.  Based on my observation of actual radar, the placement of targets on a scope is based completely on slant range.  Map items are based solely on distance from the sensor.  When I first observed this, I was confused about the discrepancies which it would introduce.  But the resolving factors are the limits on airspace dimensions and the fact that pilots are able to fly altitudes which are consistent.  We never need to worry about seperating an aircraft at 20000 feet and 4 miles away from an aircraft that is 3000 feet and one mile away.  The ranges are far more horizontal than they are vertical, and this minimizes the issues with separation.

Welcome to Aviation SE. While your answer might be correct in the sense that it is not inaccurate, it does not answer the question itself. You might want to take a look at the help.

Yes, this does answer the question.  The slant range data, projected onto a screen, represents a specific type of projection from 3-d to 2d.  Which is the point of the question.  The oldest radar units were pure analog and just showed reflection intensity on an analog CRT, and were simple time of flight vs antenna azimuth plots.  That would be a type of projection.

Contemporary systems integrating secondary data (transponders, ADS-B) with primary return data generally have processing to perform orthorectification which would make slant range arguments moot.  But having said that, the number of installations which will display slant range still far exceeds those which have a more planimetrically corrected display.

This is not comprehensive, nor does it cover mappings of radar near the polar regions, but in CONUS and many other areas, the radar screen is approximately a Lambert projection.  In reality, accurate measurements from ATC displays are not really needed, so there is little concern about the actual projection.
In targeting systems, the mapping is more closely managed, as part of the error budget in managing the targeting and navigation to the target (as in a missile guidance application).
Back to ATC, it approximates a Lambert, but the precise linearity of the projection is not critical on the screen.
Finally in center operations and in large area surveillance operations, displays covering large areas are composite displays, and once again, precise measurements are not taken from the screen, but rather from target metadata maintained by the computers.

There are three issues that must be considered in your question.  Normally the projection used for paper chart plotting considers the maximum error that printing technology normally handles.  This is, considering the thermal paper deformations due to heating sources/sinks and the mechanical tolerances at printing, this means like 0.1mm tolerances are allowed.  On a normal 1:25000 chart this means an absolute error of 2.5m in the field.  The counterpart issue to be considered is the display screen resolution.  At resolutions of 4096 pixels by 4096 pixels, you have a relative error of 1/4096 you cannot fix, as a limitation of the display screen.  This can make the error far greater in absolute units as you grow larger in map extension, which makes the projection absolutely effective by burying the errors inside the other errors.
Normal proyections do diverge normally from the point of construction (or the line in case of UTM, Lambert or Straight mercator) I consider each one.
Normally, in land places, the most commonly accepted (now) for its precission is UTM, which is not continous, as it consists in mapping the earth surface as a eliptical cilinder, with axis perpendicular to the rotating axis of the earth.  In this cilinder, only a six degree huse in longitude is considered and the plane is tangent to a complete meridian.  The maximum apart is 6 degree in longitude which makes it to deviate from the true point on less than 0.05% (and by that reason it gets scaled by 0.9995 to get half the deviation all the plane extension)  Normally, a mapping of this class completely fits in the resolution of a normal display (even at resolutions of 4000x4000 pixels) without making any point to divert by one pixel apart from the surface of reference.  This makes for a 0.03% max at aprox. 350km apart from the meridian.
Lambert, which uses a parallel of tangency, even having more diverting errors, is also far below the limit for a 4000x4000 pixel screen, even from departing from the parallel of tangency by more than 1000km.  This makes an effective exact screen for maps with more than 2000km (N/S) coverage.
Finally, polar stereographic is normally used only for high latitudes (near the poles) and is not considered in most of the globe.  Due to the elliptical surface of the earth is normally not used for representation at one point, due to the different curvature radii of the earth surface in the meridian and parallel directions.
Gnomonic projection has also been mentioned, for the properties that all the maximum circles map to straight lines (this makes normal straight approximations to map to straight lines) has similar deviations (like double, by not being a tangent line projection, but a tangent point projection it deviates more, but errors aprox. double the ones you have in tangential line projections)
It should be considered also that in case of a radar system, probably the most exact projection to achieve the best results should be a local UTM projection (with the central meridian passing through the radar rotating axis) as then you don't have any deviation from the north pole due to meridian convergence.  It is remarkable that with this remarks, probably the differences between astronomical (the ones that you measure to the geoid) and geodesic coordinates begin to be significative at ranges well over 1000km.
On other side, radars make errors and you get on the screen normally points referenced by a distance and azimuth.  As it has been said on other responses to this question.  Distance is subject to meteorological perturbations that make errors to completely overrun the ones mentioned previously. And with angles the problem is even greater, as you can be affected by refraction issues in radar.  This makes that some point acquisition be affected by errors far larger than the ones made in the projection (except in the case the projection is badly calculated)  you can trust completely the used projection.
CONCLUSION
Only in the case your projection is mistakenly selected or calculated, that you have to worry about the errors introduced by (at any range) for radar positioning.  If the projecting software is well configured, the errors will be several orders of magnitude below the measurement ones and you can consider the earth surface as almost flat.  (you have to make only the azimuth corrections due to meridian convergence anyway, as the projections are conformant only locally, but everithing you'll see in the screen will be exact as measured on the screen with the screen resolution capabilities)
CLARIFICATION
As a software engineer, the most probable chartographic projection you have will be lambert conical or UTM mercator, independently on how then the coordinates are represented, as both combine the exactitude of the results with the conformant of data grids.  But I recommend to read the radar screen manual to get the final response.  UTM  is official in the majority or countries in the world, but historically, lambert has been used for long time.

Welcome to Aviation. SE. I'm sorry, but while you provide interesting background to the problem of map projection, I don't see how this is answer the question on an ATC radar screen, which projection is typically used?


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What wing spacing is best for a tandem wing configuration?
I want to build an airplane but I have limited power(25-28hp). This means that I have to build a power efficient airplane and I finalised on building a tandem airplane. I've read that the wing spacing(distance between the trailing edge of front wing and leading edge of aft wing) should be about 30-50% of the wing span.
So my question is, what wing spacing is best?
I suppose that will help me estimate the length of the aircraft from prop spinner to rudder as intend using wing tip rudders

No. A light single seat airplane

Are you saying that you intend to use a rudder placed at the tips of the wings?

Yes. I prefer wing tip rudders.

@PeterKämpf  do u mind sending me your email address so I can very well discuss my project with you

Use my name + the usual Gmail domain. I assume you know how to handle umlauts.

Thanks a lot @ peter. I didn't go on to build the tandem plane. I realized I wasn't yet ready to build an ultralight/private aircraft yet. I haven't revisited the idea since then. Maybe sometime later but I know that I would love to build an aircraft for myself.

A tandem wing is never the most efficient planform, but makes for a compact airplane. A case in point is the French design Pou du ciel ("sky louse") which was a very popular homebuilt in the 1930s. The closely coupled wings and the short wing span made it one of the smallest man-carrying designs, but since the rear wing could act like a slotted flap, producing nonlinear lift increases, the aircraft needed careful handling.
Pitch control was achieved by changing the incidence of the forward wing, and a trailing edge down movement normally created a pitch-up moment. Once the gap between the forward wing's trailing edge and the rear wing became too small, the rear wing would show a nonlinear lift increase and add a nose-down movement which became stronger when the pilot pulled the stick further aft. In this condition the aircraft would dive into the ground unless the pilot counter-intuitively moved the stick forward again.

Mignet HM 14 Pou du Ciel (picture source). The initial type flew with a motorcycle engine of 17 HP.
Your idea of moving the wings apart is sound. However, in order to make the plane statically stable, the rear wing needs to produce less lift per area relative to the forward wing, making it less efficient. If you now run an optimizer which varies wing area and minimizes overall drag, you will invariably end up with a conventional design in which the rear wing has 15% - 20% of the area of the main wing.
What is the optimum distance between the wings? This depends entirely on your preferences. You need to know that stability increases linearly with wing-tail distance while pitch damping increases with the square of it. If you want an agile aircraft, keep both close together. If you want to reduce the tail surface and don't intend to fly aerobatics, move both apart. The key figure for stability is the tail volume, which is the product of tail surface area and the distance between wing and tail. Look at airplanes you think are worth emulating and try to match the tail volume of your design to theirs. But don't use a magic number.
In order to minimize engine power, reduce speed. The power need of an aircraft scales with the third power of speed, and many designs of the 1920s successfully used motorcycle engines. A high aspect ratio and a low wing loading, like in a glider, will help to get airborne with just 25 HP. Successful designs with such small engines were the Daimler L 20 or the Messerschmitt M 17.

Messerschmitt M 17 (picture source)

Another reason for the tandem layout is because I need a small and compact airplane with short wing span. A monoplane will fly too fast because of the high wing loading. A tandem airplane I suppose should fly a little bit slower with the same wing loading and consequently having shorter takeoff roll and landing distance. I intend to use equal span and cord wings with the front wing having an AOA 4-5 degrees higher than the aft wing. The cg will be closer to the front wing. Assume the cg of both wings as 25% of the wing cord, then the cg of the airplane will be placed at 25% of the distance bet

@user16656 The tandem wing of the same wing loading will not achieve the same maximum lift coefficient, so it will have a higher maximum speed. Note that all modern low-power designs are monoplanes (look at human-powered airplanes for examples) of high aspect ratio.

I don't know why but this configuration has always appealed to me. Fits my eyes. I have been tinkering with this design making my own adjustments and there is a lot that can be done. Just a thought.
Good luck!

Thank you. I do love the quickie too but I kinda prefer the single seater. I don't know why I just like "small"

Your answer is a good illustration but I fail to see how it answer the question ("what wing spacing is best? "). You may add some figures to your answer so that it is clear you address the asked question.

Look up the ultralight dragonfly which is a single-seater version of the quickie he may have some insight on his design and maybe the research on the quickie will help you with the spacing that you're looking for


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Can a US pilot fly a German D-xxxx aircraft in Germany?
Can a pilot with a US FAA certificate and a proper rating legally fly a D-xxx German-registered aircraft within Germany? How about the rest of the EU? Does it matter which type of aircraft? I'm most interested in gliders, but the question is more general.

No. A US licenced pilot can not fly a German registered aircraft in Germany.
Generally you must always have a pilot licence issued by the state that the aircraft is registered in.
Obtaining a licence for that country can be relatively easy based on the fact you already have a licence from another country, but you will need a new licence or some kind of documentation issued by the governing country.
There is an FAA rule that allows a US licenced pilot to fly a foreign registered aircraft the USA, but I think this is rather unique as I believe no other country allows this.

The wording of your answer may be a little misleading. A US pilot can indeed fly foreign aircraft but only inside the US, which admittedly isn't the scenario here. And if country A validates a pilot's license from country B then that pilot can fly an aircraft from country A. But what "validation" means is up to each country, so the answer here depends on whether or not Germany will validate an FAA license and if so, how.

I wasn't aware of the FAA rule that allowed flying foreign aircraft. I edited my answer. Thanks.

I find this a bit illogical, because a German pilot can fly an N-registered aircraft in German airspace. Usually this things are covered by bilateral agreements.
Validating a US license in Germany is a long, bureaucratic and painful process.

The UK also allows this. Any ICAO license is automatically "rendered valid" for flying G-registered aircraft without the pilot having to take any action.  I've used this to rent and fly many British aeroplanes on my FAA private certificate.


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What is the reason of having an Angle of Incidence on an airplane?
I know that the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the fuselage. I know that it doesn't change in flight because it is fixed.
And usually, there is a positive angle of incidence on airplanes, which means that the wing kinda looks like this:

credits: Naval Aviation Schools Command aerostudentguide(dot)pdf
My question is, why is there a need to incline the wing like that? Why is there a need for Angle of Incidence?

Strictly speaking, there is no need to set a particular angle of incidence- the wing will decide that for itself depending on the conditions (speed, weight, altitude etc.). What we are deciding is the The mounting angle, which is set for various reasons and is not variable in flight (except for some rare cases) including:

Mostly this is set so that the fuselage is (nearly) horizontal during cruise. This is especially important for airliners- DC-10, especially flew at a pronounced nose high attitude requiring the cabin crew to walk uphill.
Setting the wing at an incidence helps improve the visibility- this is important especially for carrier aircraft, where the pilot requires good visibility and also higher angle of attack (for keeping t/o and approach speeds low).
Setting the wing at an angle helps in keeping the drag low for the given lift. Having the wings at an angle and fuselage horizontal means that the drag is minimized, while the wing has the required angle of attack.

Though the angle of incidence is usually fixed, it can be varied in response to specific requirements. A good example is the Vought F-8 Crusader, which allowed the wings to be pivoted 7° out of the fuselage on takeoff and landing, resulting in a high angle of attack -- reducing the approach and take-off speed -- while keeping the fuselage level and giving the pilot a good forward field of view.

F-8 Crusader with the wing pivoted up during landing. By USN - U.S. DefenseImagery photo VIRIN: DN-SC-88-06695, Public Domain, Link
Another example is the Martin XB-51 which had a variable incidence wing to reduce the takeoff run.

This very good answer might be further improved by changing the first sentence to something like "Strictly speaking, there is no need to set a particular angle of incidence- the pilot may place the wing at the appropriate angle-of-attack to the airflow to obtain the desired performance parameter (airspeed, climb or descent rate, etc) by deflecting the elevator as needed, regardless of the incidence designed into the wing"

The wing will pick the angle of attack that is appropriate for the given speed, air density and the needed lift. If it is too high, the aircraft will accelerate upwards which will reduce the angle of attack, and vice versa. The wing's angle of incidence will then define the angle at which the fuselage sits during flight. What a proper angle is depends on the aircraft:

On transport airplanes, it makes sense to have a horizontal floor during cruise so the flight attendants don't have to push their carts uphill. This will result in a slightly positive incidence angle.
On high performance airplanes the incidence is set to optimise the lift to drag ratio.
On aerobatic airplanes it is set to zero so the aircraft can be inverted without elevator input.
On crop dusters, it might be important to give the pilot the best possible view so he/she can fly safely while manoeuvring close to the ground.

The next important detail is the height of the tail surface relative to the wing's vortex sheet. If the tail is too far above, it will sink into the wing's wake during stall and become less effective. A deep stall stability condition might be the consequence. Only by defining the angle of incidence can the airplane be built and operated as designed.

My understanding is this.
One of the design consideration of fuselage design is to avoid flow separation. And another important thing is that usual looking fuselage is not very efficient at providing lift because both above cases produce huge amount of drag.
So it is important to avoid the fuselage from separating flow and non-lifting/less lifting in normal configuration in cruse condition. To do so its important to keep the fuselage nearly horizontal
Secondly to produce max L/D or nearly max L/D wing has to operate in some positive AoA.
This can be achieved by introducing this incidence angle.

+1. The correct answer. You could improve slightly by explaining AoA abbreviation and a lift vs AoA curve/graph.

Let us denote $\delta_{wing}$ the incidence of the wing.
For moderate values of the angle of attack of the airplane $AoA$ the lift coefficient of the wing has a linear behavior :
$$Cl_{wing}=\frac{dCl_{wing}}{dAoA}(AoA+\delta_{wing}-AoA_0)$$
Where $AoA_{0}$ is the value of $AoA+\delta_{wing}$ for which the lift coefficient is zero. $AoA_0$ is a constant. Basically the incidence of the wing increases the effective angle of attack of the overall wing.

what we call THE angle of incidence of the plane is the angle between the velocity vector of the aircraft and the longitudinal axis of the aircraft. It is often denoted AoA The question clearly states angle of incidence ON airplanes not OF airplanes

For me it is unclear, aerodynamicists call this the setting angle. In the first sentence @Trekslofar says :
"I know that the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the fuselage." Which said like that sounds completely wrong.

never heard of "setting angle", always read about "incidence angle" and Trekslofar's definition is correct.

While studying for my degree this was referred to as the angle of incidence and the angle you describe was referred to as the angle of attack. Never heard of setting angle.

I did part of my degree in French (as I am French), my apologies. What I am sure of is that we call it "calage de l'aile" in French. I mixed Angle of Attack and what you call incidence angle. I consequently edit my post.

The wording is similar and they are easy to confuse even in English.

You understand that lift is generated by the action of the wing pushing air molecules down as it moves forward, requiring an angle of attack to the air passing under the wing? So if you don't tilt the wing with respect to the body of the aircraft, then to get the angle of attack required to generate lift, you would need to tilt the body as well, greatly increasing the (non-lift generating) surface area and so greatly increasing the drag.

Angle of incidence is required to generate lift if the roll axis is parallel to the airflow. Otherwise the fuselage would also be required to present AoA in order to generate lift. The wing's AoA = AoA of roll axis + angle of incidence and AoA is the primary creator of lift, not the shape of the wing, contrary to popular belief. Otherwise how could planes fly inverted?

The fly inverted in exactly the same way they fly right side up - by accelerating air downwards and proving Newton right once again.

they fly inverted by having negative AoA, whose value depends on the shape of the wing and the incidence angle. the shape has a massive influence.

"AoA is the primary creator of lift, not the shape of the wing, contrary to popular belief" So why do they spend so much money in wind tunnels when a protractor would be sufficient?

Angle of attack is required for a symmetric airfoil to generate lift. It is not required for an asymmetric airfoil to generate lift. Those can indeed generate lift at 0 AoA. Of course, you probably don't want one of those on a plane that is designed for actual inverted flight (e.g. fighters and aerobatic planes.) Note that the barrel rolls you can do in any plane aren't actually inverted flight; the wing is still flying right-side-up relative to the airflow. Of course, more AoA will still normally generate more lift on an asymmetric wing, just as it does on a symmetric one.


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Why maneuverability and top speed are still issues in 5th generation air superiority fighter jets
In a previous question someone asked why do modern fighter jet speeds decrease compared to older fighter jets, and someone hinted that nowadays top speed is not a factor considering modern rockets which will anyway exceed the maximum G forces that a human body can withstand.
Now, in the F-22 vs T-50 comparisons many hint at superior maneuverability and top speeds of the Russian T-50 design.
Shouldn't modern fighter jets be all about BVR (beyond visual range) capabilities and thus speeds and maneuverability be "surpassed".
I imagine modern fighter jets to be basically bricks in the air with a lot of high tech long distance warfare technology.

Your imagination is incorrect. How far  away would two stealthy jets be when they detect each other?

Also a consideration: what kind of fighter jet are we discussing here? Built for what purpose?

I asked with 5th generation fighter jets in mind and with air superiority in mind. Whatever distance they are I suspect a "5th" generation air to air missile will be hard to be avoided by a human manned aircraft.

Fighter jets are not bricks with powerful radar and long range missiles. BVR combat is an extremely complex affair conceptually, operationally and technologically.

For BVR combat to be initiated, the enemy aircraft should first be identified and sorted as a threat. However, assuming that both have similar radars, the inverse square law ($S \ \propto \ \frac{1}{R^{2}}$) means that you are exposing yourself to the enemy before identifying him. Combat between two stealth aircraft (F-22 vs T-50) is not unlike two men with a gun and flashlight in a dark room, as someone noted. The first one to switch on the radar is actually at a disadvantage and against a highly trained enemy, possibly suicidal. Also, let's not forget SAM ARM missiles that can home on the aircraft radars.
Though the number of aircraft shot down by radar guided missiles (as opposed to IR missiles, which are usually fired WVR) have risen significantly both in numbers and also as a percentage (of total aircraft shot down), the probability of achieving a kill by a BVR missile is still uncomfortably low. For example, the following table shows the ratio of 'kills' by radar guided missiles post WWII.

Table from Promise and Reality: Beyond Visual Range (BVR) Air-To-Air Combat by Lt Col Patrick Higby, USAF

The high percentage of BVR kills in Gulf war is basically due to two factors- AWACS and IFF(Non-Cooperative Target Recognition (NCTR) system), which gave sufficient confidence to permit BVR shots. It is highly unlikely that such a superiority level and confidence is available in case of war between two peers.
Another thing is that barely any of the BVR shots are fired head on due to the high closing speeds. So in all probability, you have to maneuver yourself in a position where a tail (or an above) shot is possible.
Also, if some missile is fired at you, speed and maneuverability are critical to carry out evasive maneuvers.
Most of the combat aircraft use in present day has nothing to do about air superiority. They are mostly used as precision guided bomb trucks. Optimizing fighters for BVR combat sacrificing other parameters is, at the least, foolhardy.

So far, the most important contributors to BVR victories (in Gulf War, for example) are not due to some inherently superior technology, but due to human factors- pilot skill and opponent's weakness. It is potentially catastrophic to generalize the doctrine against a near-peer opponent.

Good answer, still your table of BVR shots is 90's history, I was asking with 5th generation fighter jets and air superiority in mind. I think a lot has changed in the last years... I'll update my question to underline that.

@dendini, what makes you think that a lot has changed to improve the chance of BVR kill? If anything, stealth aircraft are harder for the BVR missiles to lock on.

Modern fighter aircraft will be in service for at least 20, if not 40 years. This means they will see several updates, will carry several generations of weapons and be used in unexpected, unforeseen scenarios.
Already today many combat aircraft designs are older than the pilots flying them. The relentless increase of weapon systems costs will mean that in future a smaller number of designs need to cover all demands, and do so for many more years with fewer platforms. Not giving those designs all capabilities which we can put into them will translate into a platform with clearly identifiable soft spots, and hostile weapon systems and tactics will try to exploit them to their advantage.
Neglecting one of top speed, range, endurance, maneuverability, stealth or survivability will create a deadly weakness - maybe not within the first years those designs are fielded, but certainly for the majority of their lifetimes. Just consider the variety of tasks for a modern air superiority fighter:

Intercept intruders. What is a fighter worth that cannot climb up to an unknown threat in a hurry? With fewer aircraft around, the likely intercept distance will go up, too.
Long-range strike. Let's face it, the F-15 and F-16 are used for strike missions a lot today. You have much more freedom in picking your base if the aircraft's range does't restrict you.
Combat air patrol. You need to establish permanent air superiority with as few platforms as possible. This needs endurance.
Dogfighting. In a low-intensity conflict the political directive is to visually identify the target in order to avoid embarrassing mistakes. You cannot always count on using BVR missiles.

The old black-and-white world of the Cold War has changed into a multi-faceted world with many shades of grey, and needs versatile, flexible platforms.

It is a legitimate question that is difficult to answer because there are little to no examples from actual warfare. What would happen when two pilots fight for real with $100 million aircraft on the line? It's hard to answer this definitively.
A lot will depend on the combat context. Are we fighting over the ocean or in a big mountain range, like the Caucasus or Carpathian mountains? If it is useful for an aircraft to hide in the mountains, maneuverability can be extremely important.
Speed is always very important because more speed increases the ability of the pilot to position the aircraft in a favorable location preliminary to the engagement. In exercises this matters less because both aircraft are placed in neutral or "equal" positions. In real combat usually positioning is unequal and whoever can get to the good spot first will have an advantage, often a decisive advantage.
Firing a missile at someone you cannot see is problematic because how do you know they are the enemy? Also, they can see your missile on radar way ahead of time and evade it unless you get sufficiently close before firing. In real combat the enemy will not loiter out in the middle of the wild blue wonder just waiting for you to come and shoot at them. They will always position themselves in a secure location, such as close to the ground, where you can't see them.
The defender will have a huge advantage if they have ground radar or an aerial radar, such as one on an aerostat, because they will see you coming and they can position their aircraft to ambush you. This is why SEAD is so important. Against any opponent that has anywhere near your capability, it is suicidal to act due to the advantage fixed radar will provide. In a future war it is likely that specialized antennas will be made that are distributed and difficult to destroy. This will make aerial attacks on foreign soil very difficult, unless the enemy has primitive technology.
BVR Issues
As far as the design of the T-50 is concerned, the focus on maneuverability is reasonable, especially for a defensive, air superiority fighter. As far as aircraft radar is concerned, be aware that it can only see (at best) in an 120-degree cone directly in front of the aircraft and if you use it, you light yourself up like a Christmas tree and everybody will know exactly where you are. Therefore, in a real war aircraft will, in many situations, not even use their radar, because it will give away their position, thereby allowing an enemy to pop up behind them and shoot.


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What is the difference between Boeing 777 aircraft engines and Apollo rocket engines?
What is the difference between Boeing 777 aircraft engines & Apollo rocket engines.
Do these two types of engine work the same way?

Did you read the Wikipedia article on rocket engines, particularly the introductory portion and the section on a rocket engine's principle of operation? Compare for example the second paragraph in the introductory section of Wikipedia's article on jet engines, as well as the section on rockets.

Before asking on this website, you should perform basic research so that your question is quite more specific. A quick search provides you the fact that a B777 uses jet engines (as for most -if not all- commercial long haul airliners) whereas Saturn V (the Apollo rocket) uses rocket engine (as for all rockets).

One is a jet turbine engine, the other is a rocket engine, they are completely different.

Where does one begin to answer this, each first stage engine in their own right represented the single most powerful engine on the planet. First stage F1 engines were designed to operate up to 70-80KM, 777 turbofans  cannot operate to even half this altitude.  At MECO, those first stage engines are a throw away piece of trash... 170 seconds to climb to 70Km and nearly 100Km down range, 777 Engines.... 50K hours is possible. The rocket carries an oxidizer that has been compressed and cooled prior to fuelling the tanks a jet engine sucks in the air and compresses it then expands it,

What is the difference between an AK-47 and an RPG? Do these two types of weapon work the same way?

There are two main differences.
A rocket engine does not need an external source of oxygen for combustion and has only one opening - the exhaust. It gets oxygen from its own fuel supply, typically using liquid oxygen or from another oxidiser.
A jet engine, such as those used on 777s, needs to draw in external air to get oxygen for its combustion and has two openings, the exhaust and the inlet for air.
Both use combustion of fuel to push hot expanding gases from the exhaust and Newtons 3rd law tells us that an opposite and equal reaction to the exhaust of these gases is thrust, propelling the engine forward.
Note.  Some rocket engines, such as cold gas reaction engines, do not use combustion but the ones most people think of as "rocket engines" do.
Here is a diagram of the F1 engine used on the Apollo Saturn V launchers.

Source history.nasa.gov
Here is a diagram of a typical jet engine, the Rolls Royce Trent.

Source: rolls-royce.com

Liquid oxygen (LOX) is an oxidizer, because oxygen is an oxidizer. It is one of several possible oxidizers with different properties. Another main contender is nitrogen tetroxide (N2 O4). See also.

In rocket engines, the thrust is generated by the fuel particles - there is only combustion chamber and nozzle.
In jet engines (turbofan), the thrust is generated mainly by air flowing through fan never reaching the combustion chamber.

Sort of, but the comparison should be made with a turbojet (e.g. first Boeing aircraft, or military Bell X-14) rather than a turbofan.

A turbofan, in addition of producing gases (like a rocket engine and a turbojet) also accelerates cold air that is not used to burn fuel but produces most of the thrust (this air is called bypass flow or cold flow). We can say that in a turbofan, thrust is produced by pure cold air and hot gases are nearly only used to spin the big fan which accelerates cold air.
A turbojet, like a rocket engine, produces accelerated hot gases by combustion. This is the only source of thrust for both engines. I've added the turbojet principle at the end of this post for reference.

Rocket engine specificity
A notable difference is that rocket engines of this time were designed to be started only a few times, and for the first stage only once (the upper stage used for orbital maneuvers can be started a few times).
A rocket engine must eject gases at very large velocity to accelerate the payload up to 7 km/s, while a B777 travels at less than the speed of sound (0.3 km/s). This also creates a difference in the design and working at Mach 8 rather than Mach 0.8 is much more complex.
Saturn V used for Apollo missions had 3 stages, the first stage using rocket fuel and the other using liquid hydrogen as fuel. Let's look at the second stage engine. the J-2 worked with liquid hydrogen being oxidized with liquid oxygen, the result giving water and heat. Both propellants were drawn from cryogenic (low temperature) tanks and were hugely pressurized by turbo-pumps (8,450 kPa/1,225 psi)t rotating at 8,600 rpm.

Hot burnt gases were ejected like in an aircraft turbojet to produce thrust, albeit the ejection nozzle looked more like an enormous bell than a standard turbojet nozzle.

(Saturn V second stage J-2 engine. Source: Wikipedia)
There was no cold flow like in the B777 turbofan, air was not in sufficient quantity and no fan would have worked.

B777 turbofan principle, source

For reference:

Civil turbojet principle, source

I thought the Saturn V first stage (S-IC) used F-1 engines burning RP-1 and LOX. The second and third stages (S-II and S-IVB respectively) used LH2/LOX burning J-2 engines.

Re "...rocket engines are designed to be started only a few times, and for the first stage only once...": Not universally true, e.g. SpaceX's first stage engines, or the Shutte main engines.

@FredLarson: Ah I stand corrected, this is indeed the second stage. That was a so long time ago, I had forgotten.

@jamesqf: The main engines of the Orbiter are never restarted during the flight, it would be difficult as the external tank has been jettisoned at this end of the ascent. The orbital maneuvers are done with the OMS, including the deorbit burn, smaller attitude adjustments are done with thrusters on the RCS.

@mins: I wasn't clear on what you meant.  The engines are started once per launch, but are used for a number of launches, so restarted that number of times.